An automotive clutch device is normally provided with a release mechanism that includes a hydraulic cylinder which is actuated by hydraulic pressure produced by a master cylinder. The master cylinder is in turn actuated by pressing on a clutch pedal. The pressurized actuation oil, supplied by the master cylinder, displaces a release bearing in the axial direction via a release fork, and the release bearing presses upon a central part of the diaphragm spring to move the pressure plate away from the flywheel, thereby disengaging the clutch disk from the flywheel.
As the output power of the automotive engine is increased, the transmission capacity of the automotive clutch device has to be accordingly increased. One typical approach is to increase the spring force of the diaphragm spring of the clutch device. This in turn causes an increase in the force required to release the clutch device so that a major concern now is to minimize the effort required to release the clutch device. The force required to release a clutch device can be reduced by increasing the cross sectional area of the release cylinder and/or to increase the length of the release fork. This however not only increases the pedal stroke needed to release the clutch device but also necessitates a larger space for accommodating the clutch release mechanism. For instance, a powered actuator, which uses intake negative pressure, can be used to reduce the effort required to release the clutch device without increasing the necessary pedal stroke, but requires a large mounting space and tends to be highly complex and expensive.
It is proposed in U.S. Pat. No. 4,938,332, issued to J. J. Thomas et al., to use an annular hydraulic cylinder assembly to directly actuate the release bearing, without using a release fork. An annular piston is received in a complementary annular cylinder, and a large pressure receiving surface can be achieved without increasing the external dimensions of the release mechanism. Furthermore, elimination of a release fork allows more efficient utilization of the actuation force, and can hence reduce the effort required to release the clutch device.
In an annular clutch release cylinder assembly of this type, some bending stress is produced in the annular piston as it is highly difficult to precisely align the line of force of the thrust applied to the piston with the reaction acting upon the release bearing which comes into contact with the diaphragm spring. Therefore, the annular piston must have a sufficient rigidity to be able to smoothly slide in the cylinder so that it is not practical to use plastic material for the annular cylinder. This is detrimental to reducing the weight of the annular piston.
Furthermore, as the axial dimension of the cylinder assembly has been reduced, the tilting of the piston has emerged as a major problem. Tilting of the piston could cause seizure of the piston, and cannot be tolerated for reliable operation of the clutch release cylinder.
The annular cylinder assembly disclosed in the U.S. Pat. No. 4,938,332 requires that conduits be provided in the cylinder assembly for supplying actuating oil to the cylinder; however, the conduits prevented the annular cylinder assembly from being made as compact as desired. In particular, conduits are required to be provided in the bottom region of the annular cylinder assembly which increases the axial dimension of the cylinder assembly.